1. Field of the Invention
The present invention relates to the field of well logging. More specifically, the present invention relates to methods for formation permeability estimation.
2. Background Art
Reservoir characterization is important for oil and gas production. Knowledge of formation properties is essential for effective management of the production process. Among all formation properties, permeability (or fluid conductivity) is the most difficult to determine or predict. Permeability is a measure of the ease with which a formation permits a fluid to flow through it.
To be permeable, a rock must have interconnected porosity (pores, vugs, capillaries, fissures or fractures). Greater porosity usually correlates with greater permeability. However, this correlation does not always hold true. For example, a formation of fine-grained sands may have highly interconnected pores. However, these pores and pore channels are too small and the paths available are too restrictive for fluid movement. Therefore, the permeability of a fine grained sand formation may be quite low.
While formation permeability may be directly determined using core plugs in the laboratory or using a formation fluid tester in a wellbore, these measurements are only practical for providing discrete points of permeability values. Therefore, there has been an intense interest in using models to predict formation permeability profiles from other well logging data.
Many investigators have attempted to grasp the complexity of permeability function into a model with general applicability. Most of models developed are empirical models based on correlation between formation permeability, formation porosity and irreducible water saturation. See e.g., Balan, B. et al., xe2x80x9cState-Of-The-Art in Permeability Determination From Well Log Data: Part 1xe2x80x94A comparative Study, Model Development,xe2x80x9d SPE 30978, presented at the SPE Eastern Regional Conference and Exhibition, Morgantown, W.Va., Sep. 21, 1995.
The empirical models typically involve measuring porosity and irreducible water saturation of the core and developing mathematical models relating porosity and irreducible water saturation to permeability. In order to use this approach, it is necessary to obtain effective porosity, which is the portion of the porosity that is not isolated and is connected to the pore network and therefore can contribute to fluid flow, and irreducible water saturation. These parameters are not available directly from well log. Instead, they are estimated from other well log data. However, porosity derived from well log data is not necessarily effective porosity, and methods for deriving irreducible water saturation often rely on effective porosity. Furthermore, empirical models developed for certain formation perform poorly when used in other field. See Mohaghegh, S. et al., xe2x80x9cState-Of-The-Art in Permeability Determination From Well Log Data: Part 2xe2x80x94Verifiable, Accurate Permeability Predictions, the Touch-Stone of All Models,xe2x80x9d SPE 30979, presented at the SPE Eastern Regional Conference and Exhibition, Morgantown, W.Va., Sep. 17-21, 1995.
It is desirable to have methods for providing more accurate formation permeability profiles, which match formation log data obtained from fluid flow measurements. In addition, it is desirable that such methods are generally applicable to various formation permeability models.
An aspect of the invention relates to methods for calibrating a formation permeability model with respect to data obtained from fluid flow measurements. A method according to the invention includes determining a correlation coefficient for a porosity-irreducible water saturation relationship; determining a leading coefficient for a formation permeability model; and determining at least one exponent parameter for the formation permeability model by minimizing a basis function that represents a difference between a formation permeability estimate derived from the formation permeability model and a formation permeability estimate derived from the fluid flow measurements.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.